1. Field of the Invention
This invention relates to a method of manufacturing a longitudinally watertight telecommunication cable having a core comprising a plurality of conductors each covered with a synthetic insulating material, the interstices between the conductors and the space between the core and the sheath being filled with a compound which prevents water from penetrating into the cable core in the direction of the length of the cable.
Cables of this type are known. In cables laid in the ground water may penetrate into the cable core through defects of the sheath due to mechanical damage. Such defects may be due to subsidence of the ground or to mechanical force. In the core of a cable composed of conductors or bundles of conductors which are twisted together, as generally is the case in telecommunication cables, water which has made its way into the cable may spread in the direction of the length thereof through the interstices between the conductors and the space between the core and the sheath. When the conductors are individually insulated with a synthetic resin, the presence of water becomes manifest only after the water has penetrated over a comparatively large distance, so that the electric properties of the cable deteriorate, which deterioration can be corrected only with great difficulty. Penetration of water can be prevented if highly viscous compounds or a foamed synthetic resin is applied throughout the length of the cable or in blocks, i.e. in longitudinally separated regions spaced at regular intervals in the direction of the length of the cable over a given distance.
2. Description of the Prior Art
The highly viscous compounds proposed for preventing water migration usually are paraffin-like substances, petroleum waxes, petroleum jelly and the like. These substances are melted or heated until their viscosity is low and thereafter are pressed into the interstices between the conductors and into the space between the core and the sheath. After solidification the compounds introduced into the cable form a seal against water. A disadvantage of these substances is, however, that they become liquid again when the cable temperature rises. This may occur, for example, when such a cable wound on a drum is exposed to the sun for a long time. By the action of gravity the molten sealing compound then flows to the lower parts of the cable on the drum, causing poorly filled interstices or voids to be left in the remaining parts of the cable. This is particularly detrimental when the sealing compound has been applied in blocks. Another disadvantage is that the quality of conductor insulations of polyethylene when brought into contact with some paraffin-like substances deteriorates in electrical and mechanical respects in the long run.
When the interstices between the conductors and the space between the core and the sheath are filled with a foamed synthetic material, difficulties of another nature arise. Foaming is a process which is not readily controllable and which usually starts immediately after the foam-forming mixture has left its container. Foam-forming may have stopped before the interstices between the conductors and the space between the core and the sheath have sufficiently been filled. Another disadvantage is that as a rule a cable in which the core contains a foamed synthetic material as a sealing means will be stiff, which may cause difficulties when the cable is wound on a drum and is laid in the ground.
In the manufacture of power cables having a small number of conductors of circular cross-section it is known to fill the interstices between, and the space around, the conductors to a circular cross-section with a silicone rubber which is spontaneously vulcanized at room temperature. The vulcanizing agent is added to the filler material immediately before the application thereof.
A disadvantage of this method is that when the manufacturing process comes to a standstill conduits containing the material are likely to become clogged.
For this reason the use of such a spontaneously vulcanized filler material in manufacturing telecommunication cables, in particular when the material is applied in blocks and is intended to prevent penetration of water, gives rise to difficulties.